Topic 57. Placentation. Structure of the placenta. Umbilical cord

Umbilical Cord

• The connecting cord forms the link between the developing embryo and the placenta.
• Umbilical cord at birth:
• Length: 50–60 cm
• Diameter: 1.5–2.5 cm
• Weight: 100–150 g

Formation and Development

• Develops from and contains remnants of the yolk sac and allantois.
• Forms the primitive umbilical ring during the 5th week, which consists of:
• Connecting stalk
• Yolk sac (vitelline duct)
• Vitelline vessels

Primitive Umbilical Cord

• Composition:
• Distally: yolk sac stalk and umbilical vessels
• Proximally: some intestinal loops, remnants of the allantois and its vessels
• Amnion

Definitive Umbilical Cord

• Composition:
• Wharton’s jelly
• Two umbilical arteries – return deoxygenated blood to the mother
• One umbilical vein – supplies fetus with oxygenated blood from the placenta
• Amnion

Placenta

• Organ that connects fetus to mother.
• Functions: allows nutrient intake, waste elimination, and gas exchange via mother’s blood supply.
• Full-term placenta:
• Diameter: 15–20 cm
• Thickness: 2–3 cm
• Weight: 500–600 g

Placentation

• Formation, type, structure, or arrangement of the placenta occurs from the 3rd week to the 3rd month (~12–13 weeks).
• The trophoblast is seen at about day 6–7 and ensures implantation of the egg in the uterine mucosa due to its proteolytic activity. It consists of:
• Inner layer: cytotrophoblast
• Outer layer: syncytiotrophoblast
• By week 7, the trophoblast has proliferated into villi visible over the entire surface of the chorion (placenta).
• At the end of month 2, villi begin to group together, forming the villous chorion or chorion frondosum. The umbilical cord is very thick at this stage and moves toward this region.
• By 2½ months, rarefaction of villi at one of the poles is seen.
• At 3 months, a clearly defined placenta is seen; villi are grouped at one pole (placenta), and the rest of the embryonic vesicle is devoid of villi and smooth (chorion laeve).
• After 3 months, the placenta grows, thickens, and spreads out, developing along with the uterus.
• Placenta at term: disk about 20 cm in diameter, 3 cm thick, and ~500 g (≈ one-sixth of fetal weight). Markedly different ratios indicate pathology.

Structure of the Placenta

• Fetal portion:
• Chorion frondosum
• Bordered by chorion plate
• Covered by amnion
• Maternal portion:
• Bordered by decidual plate
• Cotyledons
• Junctional zone: trophoblast and decidua cells

Functions of the Placenta

• Nutrition: transports nutrients from maternal blood into fetus.
• Respiration: transfers oxygen from maternal blood to fetus and CO₂ from fetus to maternal blood.
• Excretion: nitrogenous waste products from embryo diffuse through the placenta into maternal bloodstream.